A telephone line circuit, traditionally located at the telephone central office, provides a number of functions for the customer loop it serves. "BORSCHT" is the mnemonic frequently used to identify the principal ones of such functions: Battery feed, Over-voltage protection, Ring insertion, Supervision, Coding/decoding, Hybrid function, i.e., connection of two-wire subscriber line to the four-wire network, and Testing. In modem digital telephone switching systems, the testing of the customer loop may be performed during each call on demand in response to a complaint or on a routine/periodic basis. In a typical prior art central office switching system, such as the 5ESS Switch, per call tests are run using a "service circuit" under the control of peripheral software. Information on failures is passed to the appropriate maintenance software system. The central office switching system may perform additional tests by directly connecting a test unit to check the electrical parameters of the subscriber loop, by an automatic line insulation testing circuit or by a transmission test facility which contains DTMF testing circuitry.
The growing trend to the concentration of customer lines remote from the central office requires that BORSCHT functions be provided by line cards remotely located from the central office switch--with the result that the automatic testing equipment of the central office is no longer locally available to the line circuit. Accordingly, it would be desirable to provide a line interface circuit which could accomplish, in a self-contained manner, many of the testing functions priorly requiring the local availability of central office switching equipment. Additionally, it would be advantageous to reduce the cost and size of the line interface circuit without sacrificing reliability. In particular, it would be advantageous to eliminate the need for the electromechanical ringing relay that previously has been required to connect the ringing generator to the Tip and Ring conductors of the loop and to disconnect the remaining components of the line-feed circuit so that they would not load down the ringing signal.
It has heretofore been recognized that one of the relays in a conventional line interface circuit, namely the ringing relay, may be replaced by transistor circuitry, as in U.S. Pat. No. 4,652,701 issued Mar. 24, 1987 to R. J. Cubbison, Jr. The '701 patent employed a transistor feedback circuit to compensate for the imperfections inherent in semiconductor switches, principally the series impedance of the normally closed semiconductor switch contacts and parasitic capacitance that could couple in ringing signal when the transistor switch was inactive.
It has also been appreciated as shown, for example in U.S. Pat. No. 5,341,416 issued Aug. 23, 1994 to J. C. Gammel, that the rapid detection of the off-hook condition when the ringing signal is applied to the loop, i.e., high-speed ring trip, can be achieved monitoring the ringing signal polarity and cutting off the ringing current when it rises above a certain threshold during a particular polarity of the ringing voltage.
It would be of great advantage to provide a power-conserving, battery feed circuit which provided the usual BORSCHT functions under the control of a local microprocessor thereby permitting logic level control signals to be used. In particular, it would be advantageous to provide for a longitudinally ac balanced, current-regulated dc feed and a ringing waveform that not only permitted on-hook transmission during the silent interval of ringing but which could easily be tripped when the loop went off-hook and which required no electromechanical ringing relay to isolate the line feed from the ringing circuitry.